Statistical +/- for Houston Rockets

bydurvasa|February 1, 2010at03:11 AM

Using the statistical +/- model described at the excellent basketball-reference.com blog here and here, and data provided by dougstats.com, I calculated boxscore ratings for all NBA players who have played at least 500 minutes this season. Statistical +/- is not a +/- rating in the conventional sense, in that it is not based directly on how many points the team scores over the opponent with the player on the floor. Rather, it uses indicators in the traditional boxscore stats (such as minutes/g, points/min, reb/min, etc.) to estimate a player’s +/- impact per 100 possessions. The particular formula described in the links above was arrived at using 7 years worth of NBA data. It is not a fool-proof way of rating a player’s impact, and I don’t think such a system exists. But it serves as an alternative to PER, or NBA EFF, or whatever other method you like to use to assess a player’s worth based on his individual stats. Like any statistical basketball rating system, it should be treated as a supplement to actually watching the players perform.

Here is a list of the top players so far this season based on this statistical +/- model:

There are some surprising results for the Houston Rockets players. Lowry leads the team by far. The two rookies (Budinger, Andersen) are at the bottom, which actually shouldn’t be a surprise. Everyone else is between -1 and 1. While having a rating less than 0 is technically “below average”, keep in mind that “average” in this case is much higher than the statistical median for all the NBA players. That is, there are far more below average players in the league than above average players. An average player corresponds to a player who is basically starter-quality. If you are familiar with John Hollinger’s PER system, this would be a player with a 15 PER. Compare these results, based on boxscore indicators, to the results Wayne Winston had for the Houston Rockets as of his January 20th blog on the team. It may not be obvious from the lineup data he shows in that link, but it is possible from the information he provided to arrive at his adusted +/- ratings for each of our 9 main rotation players — both at the offensive end and defensive end. I show this graphically below:

Left to right, players go from bad to good offensively. North to south, players go from bad to good defensively (the more negative, the better). Winston’s results differ substantially from what the statistical +/- would predict for some of the players, in particular rookie David Andersen. I tend to trust the results from the statistical +/- formula more in this case. Winston has Andersen as rating well above average both on offense and defense. But Andersen, in my humble opinion, is the worst defensive player the Houston Rockets have had since Steve Novak; I can only assume that Winston’s method is allocating defensive credit to Andersen at the expense of other Rocket players (especially Landry and Budinger, who rate as awful on defense according to Winston). Winston’s results on the offensive side make a little more sense, subjectively.

Returning to the statistical +/- results, it is a surprise to see both Battier and Ariza rate so highly. The SPM model likes players who take a lot of threes (which they do), probably because such players tend to spread the floor, leading to more efficient team offense. I had thought that Ariza’s poor scoring efficiency would sink him for this rating system, but it turned out that he does enough other things well according to the model (particularly his high 3-point attempts, his steals, and his versatility index) to end up rating above average. Remember before when I said this is not a fool-proof system? I believe Ariza is a case where his offensive ineptitude is not being well-captured by this model; this is a case where Winston’s assessment rings more true.

To close, below I show a histogram of SPM ratings for all NBA players who have played at least 500 minutes:

SPM Distribution for all NBA players (Rocket players highlighted)

Again, 6 Rockets fall between -1 and 1 in the histogram above. The two rookies have less than a -2, which may after all be appropriate (though I think Budinger has shown the potential to become a starter-quality player some time soon). According to statistical +/-, Kyle Lowry has separated himself from the pack. He does not score a lot of points, and he does not shoot (or make) a lot of threes. But, by the box score, he does everything else really well. He gets to the line, he gets a ton of offensive rebounds (leads the league amongst PGs), he gets a good number of steals, and he has a high versatility index. And this doesn’t even account for the fact that Lowry leads the league in drawing offensive fouls. Has he been the Houston Rockets’ best player this season? I am not sure I would say that, but I do think he’s been the most important Rocket in regaining lost momentum from the starters’ frequent lulls to start each half. And I think many people, especially the national media at large, have under-appreciated his impact.